This invention relates to an air conditioner, and, in particular, to an air conditioner having a first refrigeration cycle involving a capacity-variable type compressor, and a second refrigeration cycle involving a constant rotation type compressor.
An air conditioner having two refrigeration cycles independent of each other is known. The compressor included in a first refrigeration cycle is comprised of a capacity-variable type compressor whose operating frequency is controlled by an inverter etc., to permit the number of rotations per unit time to vary in a stepless fashion. The compressor included in a second refrigeration cycle is comprised of a constant rotation type compressor which is normally operated with a constant number of rotations. The first and second refrigeration cycles each have an outdoor heat exchanger, but they never have their own indoor heat exchangers, i.e., they have a single common indoor heat exchanger. These refrigeration cycles permit a cooling operation as well as a heating operation.
The operation of the above-mentioned air conditioner is controlled by continuously varying the operating frequency of the compressor in the first refrigeration cycle in accordance with a difference between a temperature (T.sub.a) measured by a room temperature sensor in the indoor unit and a set temperature (T.sub.b). A control also exists to stop the operation of the second refrigeration cycle when the difference temperature (Ta-Tb) reaches a constant level. Even after the operation of the second cooling cycle is stopped, the operating frequency of the compressor in the first refrigeration cycle is variably controlled in accordance with the difference temperature (T.sub.a -T.sub.b).
In an air conditioner of this type, however, a single common indoor heat exchanger is, as described above, shared by the first and second refrigeration cycles. Thus, when the operation of only the first refrigeration cycle is performed, with the operation of the second refrigeration cycle stopped, the evaporation rate of a coolant in the first refrigeration cycle will, at the indoor heat exchanger, be about two times as great as in the case where both refrigeration cycles are operated. As a result, the evaporation pressure of the coolant is raised, resulting in an overheating cycle. In this case, the suction pressure of the compressor exceeds its allowable pressure, causing damage to the compressor, all of which poses the problem of reducing the reliability of the air conditioner. This phenomenon occurs prevalently in the case where the operating frequency of the compressor, in particular, is low.
If, under the operating conditions of the first and second refrigeration cycles, the operation of the second refrigeration cycle is stopped, a capacity gap occurs in the air conditioner between the times before and after the stopping of the operation of the second refrigeration cycle, thus presenting an obstacle to fine control of the air conditioner in accordance with a load.